This application is related to Netherlands Patent Application No. 7610557, filed Sept. 23, 1976, the entire disclosure of which is hereby incorporated by reference.
The present invention relates to a process for preparing cyanuric acid by heating urea, biuret or a mixture of urea and biuret in a solvent to produce a pure cyanuric acid product with a low content of the by-products ammelide and ammeline.
Cyanuric acid is used as an intermediate in the preparation of other chemical compounds, e.g., the preparation of chlorocyanuric acid which is used in dry bleach compositions and detergents by direct chlorination of cyanuric acid in alkaline solution, and the production of resins.
The production of cyanuric acid from urea or biuret is known. The basic steps consist of first heating urea or biuret for several hours. This results in deamination of the urea and the formation of cyanuric acid: ##STR1## The reaction, however, is not as simple as the equation suggests. When urea or biuret is heated, it goes through several states before it is converted to cyanuric acid. Initially, the starting material melts to form a freeflowing liquid. As the heating continues, the reaction mass thickens and finally solidifies. However, at this point, the reaction is not complete. The reaction mass still contains significant amounts of urea, biuret, and triuret, which require additional heating to convert them to cyanuric acid. The additional heating is difficult because of the poor heat-transfer characteristics of the reaction mass. If the reaction mass is heated to too high a temperature, the yield of cyanuric acid product decreases due to depolymerization of the product. Another problem with the reaction is that the reaction product strongly adheres to the walls of the reactor and is removed with great difficulty.
Several methods have been proposed to overcome these problems. For example, it has been proposed to run the reaction in certain high-boiling organic solvents. This and other proposed methods of overcoming the above-discussed problems are mentioned in the article "Triazinetriol" in the Kirk-Othmer Encyclopedia of Chemical Technology, Second Edition, Volume 20, pages 662 to 671, the entire disclosure of which is hereby incorporated by reference.
Many organic solvents have been proposed for use in running the reaction to convert urea or biuret to cyanuric acid. Examples of solvents which have been proposed include tetra (lower) alkyl ureas such as tetramethyl urea and tetraethyl urea; phenolic solvents such as cresol, xylenol, and chlorocresols; substituted amides such as dimethylformamide, dibutylformamide, and dimethylacetamide; glycol ethers; and methanes such as 5-methyl-2-oxazolidinone. The use of such solvents is disclosed in Netherlands Patent Application No. 74.05629 which is available for public inspection. However, each of the proposed organic solvents has its own disadvantages, particularly the presence of organic impurities in the cyanuric acid product.
It is a particular disadvantage of prior processes that the organic impurities produced in the cyanuric acid product are difficult to remove. However, the removal of such organic impurities is required for various applications of cyanuric acid. For example, ammeline and ammelide are formed as by-products in the preparation of cyanuric acid from urea, and are considered impurities in the cyanuric acid product. Typically, prior processes for preparing cyanuric acid from urea using no solvent produce as much as 20-30% impurities consisting mostly of ammelide and ammeline, with minor amount of melamine, biuret, urea and triuret (Kirk-Othmer Encyclopedia, supra). Ammeline and ammelide are very undesirable by-products because they interfere in some important applications of cyanuric acid. Consequently, the ammeline and ammelide content of cyanuric acid may be required to be less than 1% by weight. It is common practice to purify crude cyanuric acid containing ammeline and ammelide by treating the crude acid with a strongly acid aqueous solution, so that the ammeline and ammelide are hydrolyzed into cyanuric acid. However, such a hydrolysis step is expensive, so that it would be very desirable to avoid it. Cyanuric acid obtained by prior out processes wherein an organic solvent is used contains normally besides ammelide and ammeline also traces of the solvent as organic impurities which are difficult to remove.
An advantageous method of preparing cyanuric acid from urea or biuret or mixtures thereof has been discovered wherein ammonium nitrate is used as a solvent.
It is known from U.S. Pat. No. 2,949,848 that guanidine nitrate can be obtained by heating a liquid mixture of urea and ammonium nitrate at a temperature of 175.degree. to 225.degree. C. in the presence of a silica catalyst such as silica gel. The ammonium nitrate is used as a reactant with urea in this reaction. Therefore, one would not expect a melt containing ammonium nitrate to be a suitable solvent for urea in the production of cyanuric acid.
It is also known from Zhur. Prikl. Khim. 37, pages 1158-60 (1964) to prepare cyanuric acid by heating a urea melt in the absence of a solvent, but in the presence of a subordinate amount of ammonium nitrate. The ammonium nitrate is used as a quasi catalyst and not as a solvent. In such a process, the yield of cyanuric acid is very poor.